Electric actuation devices in vehicles, for example sliding roofs and window raising mechanisms, are equipped with a sensor system, which has one or two Hall sensors and interacts with a magnetic wheel connected in a torque-free manner with the rotor axle of the electromotive drive, in order to provide a Hall pulse sequence, which is used for position determination. The individual Hall pulses of the Hall pulse sequence are counted for this position determination. Furthermore the Hall pulse sequence is used in the calculation of the motor rotation speed, which is an important parameter for detection of the existence of trapping or as the case may be a fundamental input value of the anti-trap protection algorithm used in each case.
In practice the magnetic wheels employed can have production-related tolerances in terms of their magnetic distribution or sector width, so that unwelcome area displacement of up to 10% between the magnet sectors can arise.
These tolerances can be ignored for the purposes of position determination, although they are unwelcome in relation to the detection of the occurrence of a trapping instance. This is because these tolerances of sector width are fed directly into the calculation of the motor rotation speed and thereby result in a fluctuation of the calculated displacement force, which is proportional to the magnetic wheel tolerances. From this, fluctuations in the determined trapping force result, which negatively influence the stability of the system and the performance of the anti-trap protection algorithm. A method and a device for determining the actual reversal of rotation of a following rotational drive is already known from EP 1 175 598 B1. In the case of this method an asymmetrical rotor-side transmitter wheel is used, in order to provide rpm-proportional pulse sequences with intermediate reference pulses. The pulses are detected in a single stator-side sensor and analyzed in an evaluation device. This evaluation device determines the actual reversal of rotation after a completed switch of rotational direction, in particular after the completed polarity reversal of an electric rotational drive motor. This takes place by taking account of a pulse signal correction value derived from the counting of the pulse signals between in each case a reference signal before and after the actual reversal of rotation, which is detectable on the basis of a switch from a monotone increase to a monotone decrease of pulse signal lengths after a switch in direction of rotation.
A further device for determining the actual reversal of rotation of a reversing rotational drive is known from DE 10 2005 047 366 A1. This device too uses a transmitter wheel with a coding structure asymmetrically embodied in relation to the distribution along the circumference of the transmitter wheel and a single detector, which by means of scanning of the coding structure generates a rotor rpm-dependent pulse signal upon rotation of the transmitter wheel. This is fed to an evaluation unit, which determines the actual reversal of rotation through analysis of the pulse edges. The aforementioned coding structure of the transmitter wheel is formed by coding sectors of a first sector width and a pair of reference-coding sectors with a second sector width.
In order to balance the influence of the production-related tolerances of magnetic wheels with equidistant pole distribution it is already known that the pulse lengths employed for calculation of the motor rotation speed are averaged over a complete rotation of the magnetic wheel. It is disadvantageous here that as a result of this averaging over a complete rotation of the magnetic wheel and thus also of the rotor shaft of the electromotive drive, a filtering arises, which causes a time delay and further reduces the dynamics of the signal. This is disadvantageous for the anti-trap protection algorithm.